1. Field of the Invention
This invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus which includes a function for correcting image density when an image development system is deteriorated after a relatively long time of usage.
2. Discussion of the Background
In a background color image forming apparatus such as a digital color copying apparatus, an image development system is an absolutely important functional element that determines image quality of a duplicate from an original document. Among various components used in the image development system of the digital color copying apparatus, a photoconductive component, referred to as a photoconductor, is generally regarded as a key component for forming an electrostatic latent image on a surface thereof. A region of such an electrostatic latent image formed on the surface of the photoconductor has electrical potentials which differ from that of other regions of the surface of the photoconductor. Due to this difference of the electrical potentials, an electrical effect is caused such that color toner, such as, cyan (C), magenta (M), yellow (Y), and black (Bk) toner, are attracted to the region of the electrostatic latent image formed on a surface of the photoconductor. An image is accordingly visualized with the respective color toner in accordance with the electrostatic latent image, as a result.
A cycle of a copying operation conducted in the background digital color copying apparatus may be separated into three main operations, for the sake of simplicity: a reading operation in which an image of an original document is read and an image signal is generated; an image data processing operation in which image data receives various kinds of image data processing; and, a printing operation in which resultant image data is converted into a visualized image through the above-mentioned image development system onto recording paper. Among these three operations, the image data processing operation may be separated into three main functional operations: a first functional operation which is conducted right after the reading operation and which has a main function to make image signals representing a red (R), a green (G), and a blue (B) color image, respectively, on the basis of image signals output from the CCD (charge coupled device); a second functional operation which has a main function to make image signals representing a cyan (C), a magenta (M), a yellow (Y), and a black (Bk) color image, respectively, on the basis of the image signals representing the color images of R, G, and B, respectively; and, a third functional operation which is performed prior to the printing operation and which has a main function to make a signal on the basis of the image signals representing the color images of C, M, Y, and Bk, respectively, for driving an LD (laser diode) to reproduce a color image through the image development system. The above-mentioned image signals representing color images of C, M, Y, and Bk, respectively, correspond to color toner of C, M, Y, and Bk, respectively, and represent image density of these respective colors.
During the above-mentioned third functional operation, in particular, it is possible to adjust the image signals to reproduce the color image to a desired quality, or image density. Such adjustment of the image signals is achieved by multiplying data representing image density included in the image signals by a predetermined coefficient, and is commonly implemented in the background digital color copying apparatus. This multiplication is performed for every picture element of the color image for every color of C, M, Y, and Bk. Generally, a set of predetermined coefficients is provided to establish a relationship of image density between a color image of an original document, to be referred to as input data, and a color image of a duplicate from the original document, to be referred to as output data. This relationship known per se is often referred to as a gamma, and the above-mentioned set of predetermined coefficients is referred to as a gamma table.
When contents of a gamma table are expressed in a graph of X- and Y-coordinates, a relationship is shown of image density between the read image (X-coordinate) as input data and an image of the duplicate (Y-coordinate) as output data. If every point represented by the X- and Y-coordinates is linked in this graph, a plotted line is shown which may be referred to as a gamma line. A variety of line forms are ideally created for a gamma line, such as, a straight line and an S-shaped line. In addition, these lines can have various different angles of elevation to change a relationship between input and output image density so as to match various different situations of the image development system.
After having being used for a relatively long time, a color copying apparatus will usually begin to have a kind of machine fatigue on various components used therein. In such circumstances, a photoconductor particularly is a critical component since such machine fatigue causes deterioration on photosensitivity of the photoconductor with which an electrostatic latent image is created. As an inevitable result, a user sees a reproduced image in degraded quality and faces difficulties in obtaining a reproduced image of a desired quality on the color copying apparatus. Once the color copying apparatus is turned into the situation as mentioned-above, the user will be required to adjust the quality of the image, for example, by manipulating a key on an operation panel and so forth, in accordance with a predetermined manner, which is referred to as a manual adjustment.
The user can try several adjustable items, such as, for example, a level of image density in each color, contrast of light and dark in each color, and so forth. However, these efforts by the user will normally be in vain, since the user usually has little reference or experience of handling a color image. When halftone colors are taken into account, a number of possible color combinations in a color image becomes enormous, and it often becomes difficult for a user to determine which setting of color combination and halftone colors is the best in image quality. Therefore, the above-mentioned adjustments are only time-consuming for an average user. As a result, the user will eventually have to ask for a service technician from outside to adjust the color copying apparatus.
The above-mentioned issue has been addressed in a proposal disclosed in the official gazette for Laid Japanese Patent Application TOKUKOU HEI 6-69210.
This proposal describes a function of changing a gamma table to be performed to solve a problem of degraded image quality by deterioration of the image development system after a relatively long time of usage. To solve the problem, the proposed solution provides the background color copying apparatus with a plural number of gamma tables so that gamma tables operative as an initial setting of the background color copying apparatus can be changed to other gamma tables in accordance with deterioration of the image development system. More specifically, there are provided in total four gamma grades for each color of R, G, and B. A red (R) color, for example, is provided with four gamma grades from a first gamma grade of an initial gamma table to a fourth gamma grade of another gamma table. By thus arranging the gamma grades, a gamma table change for one color can be performed independently from a gamma table change for the other colors.
As an initial setting of the background color copying apparatus, each color is set at the first grade. When the background color copying apparatus begins to reproduce a degraded duplicate image from an original document, the gamma grade may be changed. Through the gamma function, the first gamma grade for each color of R, G, and B may be changed to a second gamma grade, depending upon how far a color of the image is degraded. This change can be performed until the fourth gamma grade is used for each color of R, G, and B. Thus, the user can change a gamma table for each color on the background color copying apparatus so as to set the image development system in a condition in which a color image can be reproduced to a desired quality.
This proposal further describes an appearance of a degraded duplicate image caused from a deterioration on a photoconductor, and also describes a procedure of detecting such deterioration. When the photoconductor becomes deteriorated, its photosensitivity usually becomes weakened and image density of a reproduced image accordingly becomes relatively lowered throughout a whole surface of a duplicate from an original document. In this situation, a difference of image density between one arbitrary portion of a relatively high image density in the duplicate image and the other arbitrary portion of a relatively low image density in the duplicate image will become relatively small. Therefore, this problematic situation can be detected by checking whether or not a difference of image density between two portions in the duplicate image is smaller than a predetermined value.
The proposal teaches, as a whole, a procedure of correcting the lowered image density, steps of which procedure are mentioned below. A background color copying apparatus is provided with a test mode in which correction of lowered image density is to be executed. A test sheet having an image representing three color gray scales to be read in the test mode is also provided. Each one of these three color gray scale images corresponds to one of the colors of R, G, and B, and includes four image blocks representing the respective color gray scale.
After steps of reading the test sheet and printing an image in accordance with the read image formed on the test sheet in the test mode, the printed image is subjected to measurement of image density. What is actually measured in this measurement step is a difference of image density in each color of R, G, and B between image blocks adjacent to each other. It is then checked whether or not this difference is smaller than a predetermined value. When this difference is detected to be smaller than the predetermined value, it is determined that the deterioration on the image development system is so progressed that the present gamma tables can no longer be appropriately used. In this case, it is further determined to switch the present gamma tables to the next grade so that an image can be reproduced at a desired quality as was reproduced previously. A gamma table is switched unless the present gamma table is in the last grade available. With the gamma table in such an advanced grade, an angle of elevation for the gamma line becomes greater than that of the previous gamma line. By thus applying a relatively great angle of elevation of the gamma line, the background color copying apparatus can set the image development system in a condition in which image density relatively higher than that in the previous condition can be set throughout its range from a null level to a darkest level.
However, there are problems observed at a user site in using the background color copying apparatus including the above-mentioned proposed solution. From a user viewpoint, one of the problems is present in a way such that a gamma table change is automatically executed once the user instructs execution of the gamma table change on the background color copying apparatus for whatever reasons. The reason may or may not be a deterioration of the photoconductor, which is reasonable to execute the gamma table change. However, the reason may often be that the test sheet is dirty, the photoconductor is dirty, or the like, as examples, in which case it is obviously not reasonable to execute the gamma table change. What is worse in this case is that there is no way for the user to cancel the executed gamma table change and to return to the previous condition in which the user can use the previous gamma table.
Moreover, even if the photoconductor has deteriorated and it is the right time to execute the gamma table change, it is rarely possible that the background color copying apparatus changes a gamma table so as to reproduce a color at a desired quality as was previously made, by using the gamma table change function. This is because the function of the gamma table change according to the above-mentioned proposed solution only detects the image density which is lowered, but not how far the image density is lowered from the image density which was set when an image was reproduced in superior quality. In other words, a gamma table provided in the next gamma grade can rarely be a gamma table which was set when an image was reproduced in superior quality. It is therefore obvious that the background color copying apparatus having a function of a gamma table change according to the proposed solution is not capable of properly correcting image density when an image development system is deteriorated after a relatively long time of usage.